The present invention relates to a static VAR control system and more particularly to a static VAR control system employing an asymmetrically-controlled Graetz bridge in connection with a superconducting dc coil.
Thyristor phase controlled reactors with a parallel connected capacitor bank are now used as static VAR systems to compensate for lagging load currents and to eliminate unbalanced loading of the three-phase ac system. In general, a static shunt compensator consists of three air core reactors arranged in a delta configuration and connected to a pair of antiparallel thyristors, or silicon controlled rectifiers. A three-phase capacitor bank provides a constant leading power factor. Reactor currents can be varied continuously from zero to the maximum value by proper phase control of the thyristor switches. It is known that compensators with power ratings of 20 to 100 MVAR connected to a 13.8 or 34.5 kV bus typically have 1.2% losses. These losses can be broken down into 0.15% capacitor losses, 0.6% reactor losses, and 0.45% SCR losses. In a 40 MVAR conventional static VAR control system, the total losses will amount to 480 kV.
It is well known that low frequency and dc cryogenic coils generate low losses. Therefore, a static VAR control system employing superconducting magnets is more economical than the above conventional systems. Further, VAR control requires dc superconducting coils because losses in ac superconducting coils would be too high. Thus, a direct replacement of room temperature coils by superconducting coils would not result in a system with lower losses.
A six or twelve-pulse symmetrically-controlled Graetz bridge together with a room temperature or superconducting dc magnet is suitable for VAR control. However, such a system will have high losses when the reactive power is changed quickly because the coil current also changes quickly. A superconducting VAR control system, consisting of an asymmetrically-controlled bridge and a superconducting reactor, will have low system losses because the current in the coil may be held essentially constant. The asymmetrically-controlled six-pulse bridge produces a 360 Hz harmonic in the coil and 300 and 420 Hz line current harmonics. The superconducting coil must be designed to have acceptably low losses at this frequency.
A conventional six-pulse Graetz circuit allows the use of a dc magnet for VAR control. In an ideal six-pulse bridge, only reactive power is absorbed for a phase angle delay of 90.degree.. Continuous VAR control is achieved by varying the phase delay angle for a few milliseconds, thereby changing the reactor current. Once the reactor current has reached this new value, the phase delay angle is reset to 90.degree.. During the change from one VAR value to another, real power is absorbed or fed back. If this circuit is used to compensate for fast VAR changes, the fast real power change may be an undesirable feature. A superconducting coil in this scheme would have considerable losses because of the high di/dt.
Two six-pulse Graetz bridges connected in series with a 30% phase shift between their three-phase systems result in a twelve-pulse bridge. If the commutation reactances are neglected, such a twelve-pulse bridge would be well suited for continuous VAR control by operating one six-pulse bridge in the rectifier mode and the other in the inverter mode. The individual bridge voltages are controlled in such a way that the average voltage output is zero once the coil is charged. Continuous VAR control from its maximum value to zero is achieved with the real power always identical to zero. However, the commutation reactance can not be neglected. The reactive power can only be varied continuously by about 30% without changing the coil current and coil power. If a greater VAR change is required, the average coil current must be changed, thereby causing a real power change. The twelve-pulse circuit requires a converter transformer and reduces the harmonic output of the line current compared to the six-pulse circuit. While such systems have been successfully employed, they do not easily allow for individual phase control. As in the six-pulse circuit, the superconducting coil would be subjected to fast current changes and thus would exhibit high losses when the system is installed to control fast changing VAR demands. Such prior art bridges are disclosed in W. Farrer et al., "Fully Controlled Regenerative Bridges with Half-Controlled Characteristics," Proc. Inst. Electr. Eng., 125, 2, pp. 109-112 (February 1978) and in Z. Zabar L., "Bypass Operation by Cyclic Firing of the Bridge Thyristors," Proc. Inst. Electr. Eng., 126, 9, pp 833-836 (September 1979).